Modern industrial systems and processes tend to be technically complex, involve substantial energies and monetary interests, and have the potential to inflict serious harm to persons or property during an accident. Although absolute protection may not be possible to achieve, risk can be reduced to an acceptable level using various methods to increase an industrial system's safety and reliability and mitigate harm if an event, e.g., a failure, does occur.
Integral with industrial systems are process and safety control systems, which typically include programmable controllers (e.g., programmable logic controllers (PLCs)) and a collection of sensors and actuators for detecting and reacting to events, respectively. Typically, sensors (e.g., temperature, pressure and flow transducers) are coupled to a programmable controller via signal lines that may be hundreds of feet in length. As a consequence, the potential exists for these signal lines to be inadvertently severed or short circuited during an event (e.g., an accidental collision).
During an event arising in the context of hazardous gas atmospheres (e.g., flammable gas atmospheres), electrical and/or thermal energy released from the signal lines, field devices (e.g., sensors) and/or the programmable controller circuitry may create a spark or generate sufficient temperatures to cause the gas atmosphere to ignite. Recognizing these potential hazards, regulations in the United States and Europe mandate that current, voltage and temperature be limited in electronic circuits including programmable controllers that are operating in the presence of the hazardous atmosphere.
Consistent with existing, standardized methodology, power ballast resistors are often employed to limit the amount of current drawn by signal lines. Power ballast resisters, however, are often expensive, and are typically bulky and heavy because they are intended to limit temperatures by absorbing heat with a substantial amount of thermally conductive mass and dissipating the heat with a substantial amount of surface area.